channeldb: add method to lookup a channel ID by it's funding outpoint

This commit adds a utility method whcih utilizes the edge index bucket
and allows caller to look up the channel ID of a channel by its funding
outpoint. This can be used to populate RPC’s with additional
information and also to provide users with an additional query
interface to build channel explorers, etc.

channeldb/graph.go

@@ -630,6 +630,42 @@ func (c *ChannelGraph) DeleteChannelEdge(chanPoint *wire.OutPoint) error {
 	})
 }
 
+// ChannelID attempt to lookup the 8-byte compact channel ID which maps to the
+// passed channel point (outpoint). If the passed channel doesn't exist within
+// the database, then ErrEdgeNotFound is returned.
+func (c *ChannelGraph) ChannelID(chanPoint *wire.OutPoint) (uint64, error) {
+	var chanID uint64
+
+	var b bytes.Buffer
+	if err := writeOutpoint(&b, chanPoint); err != nil {
+		return 0, nil
+	}
+
+	if err := c.db.View(func(tx *bolt.Tx) error {
+		edges := tx.Bucket(edgeBucket)
+		if edges == nil {
+			return ErrGraphNoEdgesFound
+		}
+		chanIndex := edges.Bucket(channelPointBucket)
+		if edges == nil {
+			return ErrGraphNoEdgesFound
+		}
+
+		chanIDBytes := chanIndex.Get(b.Bytes())
+		if chanIDBytes == nil {
+			return ErrEdgeNotFound
+		}
+
+		chanID = byteOrder.Uint64(chanIDBytes)
+
+		return nil
+	}); err != nil {
+		return 0, err
+	}
+
+	return chanID, nil
+}
+
 func delChannelByEdge(edges *bolt.Bucket, edgeIndex *bolt.Bucket,
 	chanIndex *bolt.Bucket, chanPoint *wire.OutPoint) error {
 	var b bytes.Buffer

channeldb/graph_test.go

@@ -335,6 +335,17 @@ func TestEdgeInfoUpdates(t *testing.T) {
 		t.Fatalf("graph should have of inserted edge")
 	}
 
+	// We should also be able to retrieved the channelID only knowing the
+	// channel point of the channel.
+	dbChanID, err := graph.ChannelID(&outpoint)
+	if err != nil {
+		t.Fatalf("unable to retrieve channel ID: %v", err)
+	}
+	if dbChanID != chanID {
+		t.Fatalf("chan ID's mismatch, expected %v got %v", dbChanID,
+			chanID)
+	}
+
 	// With the edges inserted, perform some queries to ensure that they've
 	// been inserted properly.
 	dbEdge1, dbEdge2, err := graph.FetchChannelEdgesByID(chanID)